Gas Tungsten Arc Welding (often called TIG Welding) is an arc welding process in which the heat is produced between a nonconsumable electrode and the work metal. The electrode, the weld puddle, the arc, and adjacent heated areas of the workpiece are protected from atmospheric contamination by a gaseous shield. This shield is provided by a stream of gas (usually an inert gas), or a mixture of gasses. The gas shield must provide full protection; even a small amount of entrained air can contaminate the weld.
When two pipes are welded together, inert gas is injected into the pipes to prevent sugaring or oxidizing of stainless or chrome steel pipe. "Sugaring" is a term used in the welding industry to mean oxidation of the internal "root pass," the first layer of weld material introduced into the pipe weld. Oxidation of the "root pass" is caused by a lack of an inert gas to purge a pipe of oxygen. The high nickel and chromium content of the base metal becomes reactive with oxygen in the melting process of the weld. When purge gas is initially introduced to the interior of the pipe, a groove or gap between the two pipes to be welded together is left open to the atmosphere. Approximately 90 to 95% of the purge gas, the inert gas within the pipes, will escape through this gap between the ends of the pipes. The inert gas is then displaced by air which is rich in oxygen, which reacts with the chrome or nickel in the metal of the pipe itself, resulting in sugaring.
To combat this problem, welders commonly use masking tape or duct tape to inhibit the inert gas seepage through the gap in the pipes. In some applications, such as the construction of power plants, codes (ASME Pressure Vessel Code Sections 9 and 11) exist which require that the gaps between the pipes be blocked. A common means to accomplish this blocking is the use of masking or duct tape. However, there are problems associated with the use of tape. As the metal heats up, the masking tape or duct tape leaves an acidic residue which adversely affects the pipes. An inspector will require the welder to remove the residue. Two methods exist for removing residue. The pipes can be ground to remove the residue. This is an arduous and expensive process as it requires both time and labor. Alternatively, a welder can remove the residue with acetone. While the use of acetone is a simpler process, the risks involved with acetone are great. Acetone has a low flash point, whereas the temperature of the arc on a TIG welder is significantly more than 1500.degree. F. Thus, the use of acetone on the cooling metal could have catastrophic repercussions. There exists a need for a shield which will inhibit the flow of inert gas from the interior of the pipes to be welded, without leaving a residue on the pipes.
Description of Related Art
Applicant is aware of the following U. S. Patents concerning inert gas shielding for TIG welding.
______________________________________ U.S. Pat. No. Inventor Issue Date Title ______________________________________ 3,652,818 Erlandson 03-28-1972 INERT ATMOS- PHERE SEAM WELDER 3,736,400 Speigel 05-29-1973 APPARATUS FOR DAMMING PIPE ENDS FOR WELDING 4,354,088 Rehrig 10-12-1982 GAS SHIELDED WELDING TORCH 4,828,160 Sundholm 05-09-1898 APPARATUS TO BE USED WHEN WELDING PIPES TOGETHER 4,912,293 Mueller 03-27-1990 METHOD AND APPARATUS FOR FORMING A PARTIAL PROTECTIVE- GAS ATMOSPHERE 4,912,299 Oros 02-27-1990 GAS METAL ARC WELDING OF ALUMINUM-BASED WORKPIECES 4,916,281 Flasche 04-10-1990 GAS BACK- PURGING DURING WELDING OF PIPE ______________________________________
Erlandson U.S. Pat. No. 3,652,818 teaches resistance seam welder apparatus for welding lapped sheet material, particularly can bodies. The apparatus includes means for effecting inert gas flow between roller electrode surfaces and the interface of the lapped can body edge portions. The flow effecting means being is a manifold into which inert gas is introduced, while flexible sealing means are provided for sealing contacting opposite surfaces of the can body edge portions. This apparatus differs substantially from the present invention. The cans are welded lengthwise and not around the perimeter of the can. Because of the direction of the Erlandson weld, an inert gas environment can be created around the weld spot without having to inject inert gas into the can. Therefore, the shielding in Erlandson is substantially different from that of the present invention.
Speigel U.S. Pat. No. 3,736,400 teaches a method and apparatus for welding together a pair of pipe ends. The method comprises providing a water soluble dam at each pipe end, and inserting an inert gas into the end between the dams, then welding the ends together, followed by passing a fluid containing water through the pipe to dissolve the dam. Speigel's teaching regarding use of the water soluble dams is incorporated herein by reference. However, the Speigel patent provides no means for preventing inert gas seepage from the interior of the pipes through the gap between the two pipes which are to be welded together.
Rehrig U.S. Pat. No. 4,354,088 teaches a TIG welding torch head apparatus which directs inert gas flow around the tip of the torch. This torch head design provides additional inert gas shielding, however it does not inhibit the exodus of the inert gas from the gap between the two pipes.
Sundholm U.S. Pat. No. 4,828,160 teaches an apparatus for shielding the inert gas inside pipes by use of a bellows structure, which can be pulled through the pipe and reused. This apparatus, like that of Speigel, contains the inert gas within the pipe. However, it does not prevent the inert gas from escaping through the gaps between the two pipes to be welded together.
Mueller U.S. Pat. No. 4,912,293 teaches apparatus for creating a partial protective gas atmosphere for two tubes which includes a sheath brought to the welding location through one of the tubes or through the opening of the tube in the wall of the vessel. Both of the tube ends are overlapped with the sheath or one of the tube ends overlaps with the sheath and the sheath is placed above the opening in the wall of the vessel for the other tube. This forms a chamber about the welding location. The two tube ends .are subsequently pushed together toward each other until only a slight intermediate space remains therebetween. A protective gas is introduced into the chamber through the intermediate space. the tubes then are centered in the desired position for welding.
Oros U.S. Pat. No. 4,912,299 discusses the need for shielding gas, however it does not teach any means for containing the shielding gas in any given area.
Flasche U.S. Pat. No. 4,916,281 teaches a method of welding in which inert gas flows out from a pipe interior about the weld area. To accomplish this, the patentee suggests that the weld joint should be sealed around the circumference of the pipe except for a small opening at the top position of the pipe. However, Flasche teaches no method for sealing the joint, and one can only assume masking tape or duct tape is employed to accomplish this purpose.